No digits after X.

A numeric character reference starts with &# but has no digits after it, so the parser can't turn it into a character.

Numeric character references come in two forms: &# followed by a decimal number, such as ©, or &#x followed by a hexadecimal number, such as ©. Both end with a semicolon. When the parser reads &# or &#x and the next character isn't a digit, it reports this error and keeps those characters as plain text, so &#; shows up on the page exactly as typed.

The quoted part of the message is the text that comes before &# in the same attribute value. For a reference in text content there is no preceding text, so the quotes are empty. Two common causes are a template that builds a reference from an empty variable, such as an icon font code point, and a literal &# in text or in a URL that was never escaped.

To fix it, put the code point of the character you want between &# and the semicolon, or use a named reference like ©. If you meant to show the characters &# on the page, escape the ampersand as &#.

Invalid example

<p><span class="icon" aria-hidden="true">&#x;</span> Cart</p>
<p>Type &# and a number to insert a character by its code point.</p>

Valid example

<p><span class="icon" aria-hidden="true">&#x1F6D2;</span> Cart</p>
<p>Type &amp;# and a number to insert a character by its code point.</p>

Learn more:

MDN Reference

MDN Web Docs: Character reference

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WHATWG Spec

HTML Standard: Character references

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WHATWG Spec

HTML Standard: Parsing HTML documents

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